Multi-spot scanning collection optics

ABSTRACT

Disclosed are apparatus and methods for inspecting or measuring a specimen. A system comprises an illumination channel for generating and deflecting a plurality of incident beams to form a plurality of spots that scan across a segmented line comprised of a plurality of scan portions of the specimen. The system also includes one or more detection channels for sensing light emanating from a specimen in response to the incident beams directed towards such specimen and collecting a detected image for each scan portion as each incident beam&#39;s spot is scanned over its scan portion. The one or more detection channels include at least one longitudinal side channel for longitudinally collecting a detected image for each scan portion as each incident beam&#39;s spot is scanned over its scan portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. § 120 of U.S. application Ser. No. 14/619,004, entitledMulti-Spot Scanning Collection Optics, filed 10 Feb. 2015 by Jamie M.Sullivan et al., which claims priority of U.S. Provisional PatentApplication No. 61/939,140, entitled Multi-Spot Scanning CollectionOptics, filed 12 Feb. 2014 by Jamie M. Sullivan et al. Both applicationsare incorporated herein by reference in their entireties for allpurposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to inspection and metrologysystems. More specifically, it relates to scanning type systems forinspecting and measuring semiconductor wafers and other types ofpatterned samples.

BACKGROUND

Generally, the industry of semiconductor manufacturing involves highlycomplex techniques for fabricating integrating circuits usingsemiconductor materials which are layered and patterned onto asubstrate, such as silicon. Due to the large scale of circuitintegration and the decreasing size of semiconductor devices, thefabricated devices have become increasingly sensitive to defects. Thatis, defects which cause faults in the device are becoming increasinglysmaller. Each device needs to be fault free prior to shipment to the endusers or customers.

Various inspection and metrology systems are used within thesemiconductor industry to detect defects on a semiconductor reticle orwafer. Some conventional optical inspection tools locate defects onpatterned wafers by scanning the surface of the wafer with a tightlyfocused laser spot and measuring the amount of light scattered by theilluminated spot on the wafer. Dissimilarities in the scatteringintensity between similar locations in adjacent dies are recorded aspotential defect sites. Other types of metrology systems are used tomeasure various characteristics, such as critical dimension (CD) on areticle or wafer.

There is a continuing need for improved inspection and metrologysystems, including scanning type systems.

SUMMARY

The following presents a simplified summary of the disclosure in orderto provide a basic understanding of certain embodiments of theinvention. This summary is not an extensive overview of the disclosureand it does not identify key/critical elements of the invention ordelineate the scope of the invention. Its sole purpose is to presentsome concepts disclosed herein in a simplified form as a prelude to themore detailed description that is presented later.

Disclosed is a system for inspecting or measuring a specimen. The systemincludes an illumination channel for generating and scanning a pluralityof incident beams to form a plurality of spots that scan across asegmented line comprised of a plurality of scan portions of thespecimen. The system further includes one or more detection channels forsensing light emanating from a specimen in response to the incidentbeams directed towards such specimen and collecting a detected image foreach scan portion as each incident beam's spot is scanned over its scanportion. The one or more detection channels include at least onelongitudinal side channel for longitudinally collecting a detected imagefor each scan portion as each incident beam's spot is scanned over itsscan portion.

In a specific implementation, the detection channels include a firstlongitudinal side channel for longitudinally collecting a firstplurality of detected images for the scan portions, a secondlongitudinal side channel for longitudinally collecting a secondplurality of detected images for the scan portions, and a normal channelfor collecting a third plurality of detected images for the scanportions. The first longitudinal side channel is positioned opposite ofthe second longitudinal side channel. In a further aspect, theillumination channel includes a normal illumination sub-channel forgenerating and scanning a first set of the plurality of incident beamsto contribute to the plurality of spots that scan across the pluralityof scan portions of the specimen and an oblique illumination sub-channelfor generating and scanning a second set of the plurality of incidentbeams to contribute to the plurality of spots that scan across theplurality of scan portions of the specimen.

In a further aspect, the first longitudinal side channel comprises afirst front lens arranged for receiving the first output beams that arescattered from the scan portions and directing such first output beamsthrough a Fourier plane towards a first back lens arranged for receivingand directing the first output beams towards a first sensor modulearranged for separately sensing the first output beams from the firstback lens. The second longitudinal side channel includes similarcomponents. The normal channel includes output optics for collecting anddirecting the third set of output beams towards a third sensor modulearranged for separately sensing the third output beams. In yet a furtheraspect, the first longitudinal side channel further includes a firstoptics element arranged for receiving the first output beams from thefirst front lens, spatially filtering portions of the first output beamsat the Fourier plane, and directing the first output beams to the firstback lens. The second and third optics elements include similarcomponents.

In yet another embodiment, the first, second, and third optics elementeach include an aperture having serrated teeth pointed perpendicular toan optical axis for controlling diffraction. In a further aspect, theserrated teeth of each of the first and second, and third opticselements are formed from two overlaid masks with serrated teeth so as tocover rounded portions of the serrated teeth in each mask and to formnon-rounded serrated teeth. In one example, the first, second, and thirdoptics element each include a plurality of pins that are independentlymovable to drop down into each aperture and selectively block noise,isolate signals, or block one or more diffraction spots.

In yet another embodiment, the normal and oblique illuminationsub-channels each includes a magnifier changer. In a further aspect, thenormal, first and second longitudinal side channels exclude a magnifierchanger so as to have a fixed magnification for the first, second, andthird output beams. The normal and oblique illumination sub-channelseach include a diffractive optical element (DOE) positioned after suchsub-channel's magnifier changer, and the DOE's of the normal and obliqueillumination sub-channels generate the first and second set of incidentbeams, respectively, so that the first and second set of incident beamshave a same center scan position at different magnifications. The first,second, and third sensor modules include a first, second, and third spotseparator mechanism, respectively, that are sized and positioned toseparately receive the first, second, third output beams, respectively,at a highest and lowest magnification without movement of such spotseparator mechanism. In a further example, the normal and obliqueillumination sub-channels each include a scan mechanism that isconfigured to sweep the first and second set of output beams acrossequally sized scan portions on the sample. In another embodiment, thenormal channel and the first and second longitudinal side channels eachinclude a magnifier changer to match a magnification of the magnifierchanger of the normal and oblique illumination sub-channels.

In another implementation, the first sensor module includes a first andsecond razor portion forming a first gap there between arranged toreceive a focus point for each of the first output beams and a firstplurality of prisms that are each positioned at each of the first outputbeams' focus point so as to separately receive and direct the firstoutput beams to a plurality of first fiber elements arranged toseparately receive and direct the first output beams to a firstplurality of focusing elements for individually focusing the firstoutput beams onto a plurality of first sensor elements for individuallysensing the first output beams. The second and third sensor modules havesimilar components. In a further aspect, the first, second, and thirdprisms are movable to compensate for distortion.

In another embodiment, the first sensor module includes a firstplurality of mirror and/or fiber elements sets that are each positionedat each of the first output beams' focus point so as to separatelyreceive and direct the first output beams to the first plurality offocusing elements, and the second and third sensor modules have similarmirrors. In another example, the first sensor module includes a maskhaving a plurality of apertures that each receive a focus point for eachof the first output beams and prisms or sets of mirrors that are eachpositioned at each of the first output beams' focus point so as toseparately receive and direct the first output beams to a plurality offirst fiber elements. The second and third sensor modules includesimilar components. In a further aspect, each of the first, second, andthird masks includes a grating in each aperture directing the first,second, and third output beams, respectively towards the first, second,and third sensor elements, respectively. In another aspect, at leastsome of the gratings of the first, second, and third mask haveorientations in different directions. In yet another example, thegratings of the first, second, and third mask have orientations in asame direction.

In an alternative embodiment, the invention pertains to a method ofinspecting a specimen, and the method includes (i) scanning multipleincident beams over separated scan lines of the specimen, (ii) receivingand separating output beams scattered from the separated scan lines ofthe specimen in response to the incident beams, (iii) longitudinallydirecting each separated output beam towards a sensor to longitudinallygenerate an image or signal, and (iv) detecting defects or measuring acharacteristic of the specimen based on the image or signal from eachsensor.

These and other aspects of the invention are described further belowwith reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified configuration of an acousto-opticaldevice (AOD).

FIG. 2A illustrates an exemplary dual AOD illumination system configuredto generate and scan a beam across a sample, such as a wafer.

FIG. 2B illustrates the location of a chirp packet at the end of a spotsweep of the dual AOD illumination system shown in FIG. 2A.

FIG. 3A illustrates a diagrammatic AOD illumination system thatgenerates multiple scanning spots with variable magnification, which canbe imaged using a fixed collection magnification, in accordance with oneembodiment of the present invention.

FIG. 3B illustrates the sweep positions for three spots under a lowmagnification in accordance with a first implementation.

FIG. 3C illustrates the sweep positions for three spots under a highmagnification in accordance with a first implementation.

FIG. 3D illustrates the relative sweep positions for both low and highmagnification in accordance with a first implementation.

FIG. 4 illustrates sweep positions for low and high magnification spotshaving a same scan length in accordance with a second implementation.

FIG. 5A illustrates an exemplary AOD illumination system that cangenerate multiple scanning spots without flood illumination of the AOD.

FIG. 5B illustrates the effects of changing the magnification of theilluminator using the magnification changer on the spot size, spotspacing, and scan length on a sample for the illumination system shownin FIG. 5A.

FIGS. 6A and 6B illustrate exemplary sweeps of three small spots.

FIGS. 7A through 7C illustrate how a prism can be used in conjunctionwith an illumination system to create an appropriate isolation of spotsin the collector optics.

FIG. 8 is a general diagrammatic representation of longitudinal imagingof objects.

FIGS. 9A-9E are diagrammatic representations of an inspection systemwith oblique and normal incidence and normal collection and longitudinalside collection channels in accordance with one embodiment of thepresent invention.

FIG. 9F is a side view of a portion of a system having separatebrightfield and darkfield collection channels in accordance with analternative of the present invention.

FIG. 10A is a diagrammatic side view of a spot isolator using prisms andslits for the collection system used to capture light scattered frommultiple scanning beams in accordance with a specific implementation ofthe present invention.

FIG. 10B is a diagrammatic representation of the spot separator andprisms as viewed in direction B-B of FIG. 10A.

FIG. 10C is a diagrammatic representation of a mask type spot separatorin accordance with a specific implementation of the present invention.

FIG. 10D is a diagrammatic representation of a mask type spot separatorwith gratings in accordance with an alternative embodiment of thepresent invention.

FIG. 10E illustrates a top view of a mask type spot isolator withgratings that are oriented in different directions in accordance withanother embodiment of the present invention.

FIG. 10F is a diagrammatic representation of an aperture having serratedteeth and is formed from two masks with serrated teeth in accordancewith one embodiment of the present invention.

FIG. 10G shows both ideal and non-ideal aperture teeth.

FIG. 10H illustrates a Fourier filter in the form of closely spacedpin-like structures that can be dropped down across the aperture toblock particular portions of the light in accordance with one embodimentof the present invention.

FIG. 11A is a top view representation of segmented longitudinal imagingcollection system in accordance with one embodiment of the presentinvention.

FIG. 11B illustrates one example of the scanning direction of the stageand AOD with respect to three spots.

FIG. 11C is a diagrammatic representation of interleaved scanning withXY stage motion in accordance with one embodiment of the presentinvention.

FIG. 11D is a diagrammatic representation of interleaved scanning withXY stage motion in accordance with an alternative embodiment of thepresent invention.

FIG. 12 is a flow chart illustrating a general procedure for inspectinga sample using a longitudinal system in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail to not unnecessarily obscure the present invention.While the invention will be described in conjunction with the specificembodiments, it will be understood that it is not intended to limit theinvention to the embodiments.

Introduction

Some scanning systems include an illumination system having one or moreincident beam sources for scanning or sweeping one or more beams acrossthe wafer. The scanning system may specifically include an acousto-opticdeflector (AOD) and a mechanism for controlling the AOD's deflectioncharacteristics. For instance, a clock may be used to generate a “chirp”signal input to each AOD. For example, FIG. 1 illustrates a simplifiedconfiguration of an acousto-optical device (AOD) 120. AOD 120 includes asound transducer 121, an acousto optic medium such as quartz 122, and anacoustic absorber 123. Other acousto optic medium materials, besidesquartz, can be utilized, depending on the particular wavelengthrequirements of the system. The acoustic absorber could be a cut in theacousto optic medium 122. An oscillating electric signal can drive soundtransducer 121 and cause it to vibrate. In turn, this vibration createssound waves in quartz plate 122. Acoustic absorber 123 can be formedfrom a material that absorbs any sound waves that reach the edge ofquartz plate 122. As a result of the sound waves, incoming light 124 toquartz plate 122 is diffracted into a plurality of directions 128, 129and 130.

A diffracted beam emerges from quartz plate 122 at an angle that dependson the wavelength of the light relative to the wavelength of the sound.By ramping frequencies from low to high, portion 126 may have a higherfrequency than portion 127. Because portion 126 has a higher frequency,it diffracts a portion of the incident light beam through a steeperangle as shown by diffracted beam 128. Because portion 127 has arelatively lower frequency, it diffracts a portion of the incident lightbeam through a more shallow angle as shown by diffracted light beam 130.Because a mid-section portion between portions 126 and 127 has afrequency between the higher and relatively lower frequencies, itdiffracts a portion of the incident light beam through an intermediateangle as shown by diffracted light beam 129. Thus, an AOD can be used tofocus an incoming beam 124 at position 125.

FIG. 2A illustrates an exemplary dual AOD illumination system 110configured to generate and scan a beam across a sample 109, such as awafer. A prescan AOD 101 can be used to deflect the incident light froma light source 100 at an angle, wherein the angle is proportional to thefrequency of the radio frequency (RF) drive source. A telephoto lens 102can be used to convert the angular scan from prescan AOD 101 into alinear scan.

A chirp AOD 104 can be used to focus the incident beam in the plane ofacoustic propagation onto a scan plane 105, which can be accomplished byramping thru all the RF frequencies with transducer 104A faster thanthose frequencies can all propagate thru chirp AOD 104. This rapidramping forms a chirp packet 104B. Chirp packet 104B then propagatesthru chirp AOD 104 at the speed of sound. FIG. 2A shows the location ofchirp packet 104B at the start of a spot sweep, whereas FIG. 2Billustrates the location of chirp packet 104B at the end of that spotsweep. Note that during this propagation, prescan AOD 101 can adjust itsRF frequency to track the chirp packet in AOD 104 to keep the light beamincident upon chirp packet 104B.

A cylinder lens 103 can be used to focus the beam in a planeperpendicular to the plane of acoustic propagation. A relay lens 106 canbe used to generate a real pupil at a pupil plane 106A. A magnificationchanger 107 can be used to adjust the size of the spot and the length ofsweep. An objective lens 108 can then be used to focus the spot onto asample 109, such as a wafer.

Other systems may utilize a beam expander in place of the pre-scan AODto form a “flood AOD” system. In a flood AOD configuration (not shown),a single or multiple chirp packets (not shown) can be generated in AOD104. Since the entire AOD is flooded with light from the beam expander,AOD 104 focuses the light incident on each chirp packet and, thus, eachchirp packet generates its own spot. Therefore objective lens 108focuses one or more spots onto sample 109 simultaneously (not shown).

When an AOD that produces multiple chirp packets is used to generatemultiple spots, a larger AOD is needed since each chirp packet has afinite size as a result of the time required to ramp through therequired RF frequencies. The more chirp packets; the larger the AODrequired. Additionally, each of the chirp packets is attenuated as ittravels along the length of the AOD. Thus, a larger AOD results inlarger attenuation losses than a smaller AOD. Conversely, an AOD thathas closer multiple chirp packets and, thus, scanning spots in closeproximity to one another results in more crosstalk between scanningspots

Note that sample 109 is typically placed on an XY translation stagecapable of bi-directional movement. In this configuration, the stage canbe moved so that the focused spots (formed by the focusing optics usingthe diffracted light beams) impinging sample 109 can be scanned alongadjacent contiguous strips of equal width (e.g., raster scan lines).U.S. Pat. No. 4,912,487, issued to Porter et al. on Mar. 27, 1990, andincorporated by reference herein, describes exemplary illuminationsystems including a translation stage configured to provide rasterscanning.

Example Illumination Embodiments

FIG. 3A illustrates a diagrammatic AOD illumination system 300 thatgenerates multiple scanning spots with variable magnification, which canbe imaged using a fixed collection magnification, in accordance with oneembodiment of the present invention. In this embodiment, a diffractiveoptical element (DOE) 311 can be positioned after magnifier changer 307to generate a plurality of spots. Although FIG. 3A shows three spotsbeing generated, other embodiments can generate a different number ofspots.

In the illustrated example, DOE 311 generates the three spots after theillumination beam has been magnified by magnifier changer 307. That is,DOE 311 causes three beams (shown by dark lines, light lines, and dashedlines) to form three spots 310 a, 310 b, 310 c on sample 109. FIG. 3B-3Dillustrate the effects of changing the magnification of magnifierchanger 307 on the spot size, spot spacing, and scan length on sample109 for illumination system 300. The three different spots areillustrated with different gray levels (gray, black, and white), whichcorrespond to the three spot positions 310 a, 310 b, and 310 c,respectively, of FIG. 3A.

FIG. 3B illustrates the sweep positions for three spots under a lowmagnification in accordance with one embodiment. For example, first spot(gray) moves from position 1 to position 5 during time duration T₀ toT_(n). There are positions between adjacent scan positions. Forinstance, second spot (black) scans from position 9 through position 13during time duration T₀ to T_(n). Accordingly, positions 6 through 8remain unscanned or blank during this time duration although asubsequent scan would typically be performed to scan these previouslyunscanned positions. Each sweep can be produced, for example, by the AODthat generates the initial beam sweep and the DOE that multiplies theinitial beam sweep, for example, into three sweeps. As shown, the threespots sweeps are each represented by a corresponding scan boxes 332 a,332 b, 332 c, respectively.

FIG. 3C shows the sweep positions for all three spots under highmagnification. In this example, three smaller sized sweep boxes 334 a,334 b, 334 c are produced under high magnification. Although themagnification of each spot may change, the DOE 311 that is placed afterthe magnification changer 307 causes each scan box to have a same centerposition for different magnifications. FIG. 3D illustrates the relativesweep positions for both low and high magnification. For instance, whilea high magnification first spot (gray) would be scanned from position 2to position 4, a low magnification spot (gray) would be scanned fromposition 1 to position 5. As shown, the low magnification scan boxes 332a-332 c have the same center as the high magnification scan boxes 334a-334 c. If the collection area corresponds to the largest scan boxes(e.g., 332 a-332 c) for the lowest magnification, both the low and highmagnification spots may be separately imaged by the collection optics.

In a further embodiment, the entire length of the AOD may be used forhigh magnification, while only a portion of the AOD's length is used forlower magnifications. FIG. 4 illustrates sweep positions for low andhigh magnification spots having a same scan length in accordance with asecond embodiment. As shown, the positions of both the high and lowmagnification sweeps are the same. For instance, both low magnificationand high magnification first spots (gray) scan from position 1 toposition 5, and result in same length scan boxes 402 a and 404 a,respectively. Likewise, both low magnification and high magnificationsecond spots (black) scan from position 9 to position 13, and result insame length scan boxes 402 b and 404 b, respectively. Both lowmagnification and high magnification third spots (white) scan fromposition 17 to position 21, and result in same length scan boxes 402 cand 404 c, respectively. This embodiment more efficiently covers thesegmented scan line under variable magnifications.

FIG. 5A illustrates another exemplary AOD illumination system 500 thatcan generate multiple spots without flood illumination. Both FIGS. 3Aand 5A are simplified diagrams and do not show every component that maytypically be present in such a system so as to simplify the description.For instance, there may be a relay lens located between the DOE andobjective lens. When the pupil of the illumination system is physicallylocated at the objective and inside the lens assembly, a relay istypically used to form a real pupil outside the objective so that theDOE may be placed at such pupil. For low numerical aperture systems, thephysical stop location will be outside the objective lens assembly. Forhigh numerical aperture systems the physical stop may be located withinthe objective lens assembly. In this case, an additional relay would beadded to the system to provide a location to place the DOE.

In the embodiment of FIG. 5A, a diffractive optical element (DOE) 501can be positioned before magnifier changer 507 to generate a pluralityof spots. Although FIG. 5A shows three spots being generated, otherembodiments can generate a different number of spots. FIG. 5Billustrates the effects of changing the magnification of magnifierchanger 507 on the spot size, spot spacing, and scan length on sample109 for illumination system 500. Note that the different fill grayscalelevels indicate different spots (and correspond to the different linegrayscale levels of FIG. 5A). As shown in FIG. 5B, large spots 520 havespacing associated with three positions 1, 3, and 5, whereas small spots521 have spacing associated with three positions 2, 3, and 4. The spotpositions for both low and high magnification spots in FIG. 5B representthe center scan position for each spot.

Having high and low magnification scan segments with overlap makesappropriate isolation for the scattered light from the multiple spotsmore difficult. For example, FIGS. 6A and 6B illustrate exemplary sweepsof three small spots 601, 602, and 603 (corresponding to those shown inFIG. 5B) between times T₁ and T₄. FIG. 6B represents the scans of spots601, 602, and 603, as boxes of the same grayscale level, wherein theboxes represent the paths of the spots as a result of the propagationthrough the chirp AOD. FIG. 6B shows that there is an overlap of thecolinear scans of different spots (which would occur for both the bigspots and the small spots). This overlap will result in undesirable spotcrosstalk.

To provide the isolation between spots, thereby minimizing crosstalk,additional optics and techniques can be used. In one example, shown inFIGS. 7A through 7C, a prism 700 can be used in a collection system toisolate spots that are staggered in the stage propagation direction bythe illumination optics. FIG. 7C is the prism that is located in thecollection optics. U.S. Pat. No. 7,075,638, issued to Kvamme on Jul. 11,2006, and incorporated by reference herein, describes such anillumination and collection system. In this system, prism 705 andadditional optics, such as a spherical aberration correction lens and atransmitted lens, can be positioned such that scattered light from theplurality of spots, e.g. beams associated with spots 701, 702, and 703(FIG. 7A), on the sample are directed to a specific facet of prism 705,as shown in FIG. 7C. In turn, prism 705 directs each beam to a separatedetector. FIG. 7B shows the scan sweeps of spots 701, 702, and 703during operation of the associated inspection system. Prism 705 (whichis part of the collector) takes advantage of an offset shown in FIGS. 7Athrough 7C (the offset being generated by a grating, which is part ofthe illumination system) to desirably increase the spot isolation. Thus,referring back to FIG. 7A, turning a grating will result in spots 701,702, and 703 (and their associated scans) no longer being co-linearalong the scanning axis (e.g., they will instead form 3 line segmentsstaggered in the XY stage propagation direction).

The prism in the collection optics may work for a range ofmagnifications (not fixed), even if there is no collection sidemagnification changer. However, the center scanning spot (702) willremain fixed and the side spots (701 and 703) will creep up and down thefacets of the prism (in addition to changing size and length of the scanboxes) as magnification changes if there is no magnification changer inthe collection side. If there is a magnification changer in thecollection channel that changes with the illumination magnification, theimages of the spots on the prism can remain constant.

Although the illustrated prism 705 works well under certainapplications, the prism approach is only capable of supporting a limitednumber (e.g., 3) of illumination spots. Additionally, this system'sillumination only works well with normal and near normal angles ofincidence and does not work with highly oblique angles of incidence, inwhich the objective is tilted relative to the sample (or visa versa).Oblique angled incident beams that are scanned across the sample willtend to have some of the spots not be positioned in the focal plane(unfocused) due to the staggering of the spots (e.g., FIG. 7A). Althoughan extremely large NA (numerical aperture) objective lens may be used toachieve oblique angles without tilting, a large NA lens has significantassociated costs.

The accurate detection of defects on a sample surface depends on thecorrect measurement and analysis of each spot in the scan independently.Therefore, a need arises for optimizing techniques and systems usingspot scanning techniques such as AODs that ensure the isolation of thesespots, thereby minimizing crosstalk, while minimizing system complexityand cost.

Example System Embodiments

In contrast to systems that have the objective axis perpendicular to theimage plane, certain embodiments of the present invention include alongitudinal imaging system having imaging and non-imaging optics withan optical axis that is parallel and coincident with the sample plane orimage plane. That is, the illumination spots are imaged along theoptical axis of the collection system, rather than in a plane that isperpendicular to the optical axis of the collection system.

FIG. 8 is a general diagrammatic representation of longitudinal imagingof multiple objects. As shown, two spots 802 a and 802 b that arescanned on the surface of sample 801 are imaged parallel to the opticalaxis 803 of lens 806 a and 806 b. The lens 806 a and 806 b may start aswhole lens that are subsequently cut (e.g., in half or more) so that thesample does not run into the lens as the sample moves. The optical axis803 of the lens 806 a and 806 b is coincident with the wafer surface andillumination scanning spots. Resulting image objects 804 a and 804 b arecollected by this longitudinal system.

In certain embodiments, the illumination optics are configured such thatthe scanning spots are positioned to all lie along a single line withthe spots interleaved (e.g., line—blank—line—blank, etc.). The opticalaxis of the illumination objective can be tilted and still have all thespots in focus because they all lie and remain along a single line. Theside collector is tilted such that its optical axis is the line formedby the scanning spots of the illumination optics. As viewed from theoptical axis of the collector, the spots from the illumination opticswould be scanning away or toward the observer, rather than left to rightor up or down.

FIGS. 9A-9E are diagrammatic representations of an inspection system 900with longitudinal imaging in accordance with one embodiment of thepresent invention. Although this system 900 is described as having twoincident channels for generating normal and oblique incident beamscanning spots and three detection channels for detecting light fromnormal and two longitudinal side channels, the system 900 may includeany suitable number and type of incident and collection channels,including at least one longitudinal collection channel. Additionally,although each channel is illustrated with respect to three scanningspots, each incident channel may generate any number of scanning spotsand each detection channel may detect light from any suitable number ofscanning spots. The system may generate 3 or more spots. In specificalternative implementations, 9, 15, 30, etc. spots are generated from anormal and an oblique angle and light is collected from a normal and twoor more longitudinal side channels. Each spot may have any suitableshape, such as circular or elliptical.

FIG. 9A is a side view of an inspection system 900 in accordance withone embodiment of the present invention. In general, the inspectionsystem 900 may be utilized to inspect a sample 902 for defects ormeasure characteristics, such as critical dimension (CD) or filmthickness, on the sample 902. The sample surface 902 may be smooth orpatterned.

The view of FIG. 9A shows an illumination system 901 for generatingnormal and oblique incident beams and a normal collection channel 931 afor collecting light that is reflected normal and near normal to asample surface 902. However, the system 900 also includes twolongitudinal side collection channels that are not shown in this sideview of FIG. 9A, but such side channels are illustrated in FIGS. 9D and9E.

FIG. 9B illustrates normal collection resulting from oblique incidentlight, while FIG. 9C illustrates normal collection resulting from normalincident light. FIGS. 9B and 9C are more detailed diagrammatic sideviews of the inspection system 900. However, only a single normal outputbeam is shown for simplifying the description of normal collection. Incontrast, FIG. 9E illustrates a front view of the normal collection andlongitudinal side collection of three beams.

The illumination system 901 includes one or more light sources forgenerating the scanning beams. As shown in FIG. 9A, a light source 904(typically, a laser) emits a beam 906. The wavelength of theillumination beam 906 depends on the particular requirements of theapplication. For example, the illumination beam 906 has a wavelength ofabout 266 nm. Beam 906 can be produced by any suitable light source,such as a DPSS (diode pumped solid state) CW (continuous wave) DUV (deepultraviolet) laser.

Beam 906 may be directed through zoom optics element 908, whichcompensates for laser beam changes in size, and the beam 906 thenimpinges on beam steering system 910 for aligning the beam on aparticular axis having additional illumination elements, such aspre-scan AOD 912, telephoto lens 914, Chirp AOD 916, and cylinder lens918. The illumination optics may include additional lenses, cylindricallenses, waveplates, filters, and one or more air slits.

The illumination path may include other optical elements, such as arelay lens 922 for collimating the incident beam, analyzer 923 forpolarization, waveplates 924 for providing any linear or circularpolarizations (e.g., S, P, etc.), and any number of mirrors (e.g., 920a˜d) and beam splitters (e.g., 926 a and 926 b) for forming both normaland oblique incident beams. In alternative embodiments, element 926 a isreplaced with a beam splitter, prism and/or mirror assembly. Any of themirrors or beam splitters may be movable (e.g., actuated mirror/splitter926 a).

Any number of mirrors and beam splitters may be used to form multipleincident paths. As illustrated, the incident beam 906 is transmittedthrough beam splitter 926 a, towards magnification changer assembly 927a, which is configured to change the magnification of the normalincident beam prior to it being incident on DOE 930 a, which isconfigured to form multiple beams 906 a, which are reflected frommirror/beam splitter 926 b and focused onto sample 902 via objective 928a, which doubles as a collector lens. The normal path may also include arelay lens 933 a for generating a real pupil at the pupil plane at whichDOE 930 a is placed.

The illumination system 901 may include components for forming one ormore oblique incident beams. For instance, beam splitter/mirror/prism926 a reflects a portion of the incident beam towards magnificationchanger assembly 927 b, which is configured to change the magnificationof the oblique incident beam prior to it being incident on reflectingmirror 920 d. DOE 930 b generates multiple oblique incident beams 906 bthat are focused by objective 928 d onto sample 902. The illustrated 3×1DOE elements for generating multiple beams may be replaced by anysuitable DOE or, more generally, any n×m DOE. The oblique path may alsoinclude a relay lens 933 b for generating a real pupil at the pupilplane at which DOE 930 b is placed

The optical axis (907 in FIG. 9B corresponds to one oblique beam) ofeach oblique incident beam (906 b) may be directed onto the wafersurface 902 at an angle θ. This angle θ may be in the range of 10-85degrees with respect to the normal to the sample surface 902, dependingon the particular application. Multiple oblique angles may be achievedby translation of component 920 d. Incident oblique light comes in at anoblique angle from a tilted objective 928 d.

Referring back to FIG. 9A, the scanning mechanism includes the chirp AOD916 and the translation or sample stage 921, upon which the wafer orsample rests. The position of the wafer on the stage 921 may bemaintained in any convenient manner, e.g., via vacuum suction.Inspection is achieved by scanning the chirp AOD 916 in one direction(referred to as the fast scanning direction) while moving the stage 921in an orthogonal direction (referred to as the slow scanning direction).This produces a ribbon like shape of inspected area per illuminationspot. The stage 921 then steps to a new uninspected area and repeats theprocess (filling in the blank) as described below.

Each illumination optics spot may be moved with respect to the stage soas to direct light to the sample and/or the stage moved relative to eachcollection channel, including one or more detectors or cameras, so as tocollect light from the sample by any suitable movement mechanism. Forexample, a motor mechanism may be utilized to move the stage or anyother component of the system. Each motor mechanism may be formed from ascrew drive and stepper motor, linear drive with feedback position, orband actuator and stepper motor, by way of examples.

The illustrated system 900 also includes normal collection channel 931 awhich can be used to collect scattered light from the oblique incidenceillumination mode, as well as specular or BF (brightfield) and scatteredlight from the normal incidence illumination mode. Light directed at thechannel in the normal and near normal direction may be transmittedthrough lens 928 a, beam splitter/mirror/prism 926 b, lenses 940 a and941 a, Fourier filter and configurable aperture assembly 934 a, lens 936a, and polarization analyzer assembly 932 a and be directed towardssensor module 938 a.

The normal collector channel 931 a may collect light over a fixed solidangle over a region which is approximately perpendicular to the plane ofthe wafer. The normal collector may be used to collect scattered lightfrom the intentional patterns on the wafer, as well as to detect defectswhich scatter light in an upwards direction. Signals collected from theintentional patterns may be used to facilitate the alignment andregistration of the wafer pattern to the coordinate system of themechanical stage in the instrument.

FIG. 9F is a side view of a portion of a system 980 having separatebrightfield and darkfield collection channels in accordance with analternative of the present invention. The system can have similarcomponents as the system 900 of FIGS. 9A through 9E. However, the system980 may include any number of optical elements for directing thespecular brightfield light and the scattered darkfield light thatemanates from the sample towards separate collection channels. Forinstance, mirror 982 may have a center reflective portion that reflectsthe normal incident beams towards the sample and also reflects thespecular reflected output light from the sample towards mirror 984,which reflects such brightfield light towards brightfield collectionchannel 981 a. The mirror 982 may also have transparent side portionsthat transmit the scattered and diffracted light from the sample 902towards a darkfield collection channel 981 b. The darkfield andbrightfield collection channels may have similar components as describedherein. Alternatively, these collection channels may share one or morecomponents.

As shown in both FIGS. 9B and 9C, light that is reflected or scatteredfrom the surface in the wafer surface normal or near normal direction iscollected and collimated through lens assembly 928 a. Lens assembly 928a may include multiple optical elements so as to produce a realaccessible collection pupil. This collimated light may then betransmitted through optical element 926 b and then through lens 940 aand 941 a, which may be configured to relay the collected light towardsa Fourier plane. Fourier filter and flexible aperture mechanism 934 amay be configured to spatially filter portions of the output light atthe Fourier plane. In addition, mechanism 934 a may include aprogrammable aperture system for transmitting various spatial portionsat the Fourier plane to maximize signal and/or minimize noise (andresulting angles with respect to the normal optical axis) of the outputbeam.

The output normal beams may then be focused by lens 936 a throughpolarization analyzer 932 a onto sensor module 938 a. Only a portion ofeach sensor module 938 a is shown for a single normal output beam. Asshown, the sensor module 938 a may include a spot or beam separationassembly having a slit 990 a and prism assembly 984 a for separatingeach output beam. As shown, each spot passes thru the slit 990 a andthen into a prism 984 a. The prism 984 a is used to both separate thespots and homogenize the light. The output light for each beam may thenbe output from its corresponding prism (984 a) onto a fiber opticselement 974 a for passing the output beam towards focusing element 964a, which focuses its output beam onto a sensor (954 a). Fiber opticselement 974 a provides further homogenization of the beam and enablesthe output to be directed onto a separate sensor 954 a for each spot.The function of the fiber could also be accomplished using mirrors,prisms or the like. Each fiber randomizes the received output light. Asdescribed further below, other isolation mechanisms may be used, besidesutilizing a slit, prisms, and/or optical fibers.

FIG. 9D illustrates a top view of the system 900, including the obliqueincident channel of the illumination system 901 and side collectionchannels 931 b and 931 c. As shown, the oblique incident channelgenerates three oblique incident beams 907 a (gray), 907 b (black), and907 c (white) on sample 902. The three incident beams may be sweptacross the sample, for example, in direction 903. In response to theseincident beams 907 a˜907 c, output beams that are scattered, diffracted,or reflected from the sample may be collected by side channels 931 b and931 c. As shown, the side collection channels 931 b and 931 c have anoptical axis that is parallel to the scan direction 903 of the obliqueillumination subsystem.

It is noted that the side collection channels are both shown to beperpendicular to the oblique incident channel, as viewed from the top inFIG. 9D. For instance, if the oblique incident system 901 was defined tobe positioned at a yaw angle of 180°, collection channel 931 b ispositioned at 90° and collection channel 931 c is positioned at 270°.The longitudinal collectors can be configured to collect light+/−20-80degrees (e.g., 65 degrees) in azimuth and 20 to 80 in elevation. It isnoted that only a small portion of each side output beam's cone is shownin FIGS. 9D and 9E so as to more clearly show each beam's relative path.

The system may also include any number of side collection channels,besides the illustrated two side channels that have an opposite yawangle. For instance, the system can include more than one pair ofopposite side channels. In another example, the system may include anynumber of side channels that each do not belong to a pair of oppositeangle side channels.

As shown, each side collection path may include a front lens or lensgroup (928 b or 928 c) for receiving the output beams that are scatteredfrom the sample 902 and directing such beams towards a position at whicha Fourier filter, programmable aperture or apodization element (934 b or934 c) is placed. The output beam may then be directed towards a rearlens or lens group (936 b or 936 c), which may then direct the outputbeam through a polarization analyzer element (932 b or 932 c) to befocused onto a sensor module (938 b or 938 c), which are describedfurther below with respect to FIG. 9E. The polarization analyzerelements (932 b and 932 c) may be configured for selectively capturingany polarization scattered, diffracted, or reflected from the sample902.

FIG. 9E is a diagrammatic side view of the system 900 of FIG. 9D asviewed in direction A-A. This illustration also shows the normalcollection channel 931 a, but does not show the illumination channel 931c. As shown, the front lens (928 b and 928 c) and rear lens (936 b and936 c) are in the form of half lenses although they may be any suitableportion of a whole lens so as to collect obliquely scattered,diffracted, reflected light along a longitudinal axis with respect tothe sample.

Each collected side beam is focused to impinge on a sensor module (938 bor 938 c). Each sensor module (938 b or 938 c) may include a spotseparation mechanism including the slit assemblies (990 b or 990 c) andprism components (984 c, 986 c, and 988 c) for separating the differentoutput beams (937 a˜c or 939 a˜c) into separate receiving prisms. Asshown, sensor module 938 c includes three prisms 984 c, 986 c, and 988 cfor separately receiving three output beams. The output beams bouncewithin each prism to be output towards a corresponding fiber opticsmodule (e.g., 974 c, 976 c, or 978 c), which then directs thecorresponding output beam towards a focus lens (e.g., 964 c, 966 c, or968 c), which then focuses the corresponding output beam onto a sensor(e.g., 954 c, 956 c, or 958 c). Each sensor can take the form of a PMT,avalanche photodiode, pin diode, CCD camera, etc.

Sensor module 938 b may have similar components as sensor module 938 c.Likewise, the normal collection sensor module 938 a may include similarcomponents, such as slit assembly 990 a, three prisms (e.g., 984 a),three fiber optics elements (e.g., 974 a), three focus lens (e.g., 964a), and three sensors (e.g., 954 a).

Mechanisms for increasing dynamic range of the detected signals may beprovided in proximity to collector channels. In general terms, a highdynamic range collector includes a light sensor, such as aphotomultiplier tube (PMT), for generating a signal from detectedphotons and an analog to digital converter (ADC) for converting thelight signal to a digital light signal. Of course, other suitablemechanism may be used for sensing light and converting an analog signalinto a digital signal. A gain adjustment feedback system may also beused to adjust the gain of each PMT.

It is noted that the illustrated system 900 includes illuminationmagnifier changers (927 a and 927 b), while excluding a collectionmagnifier changer. In other words, the system 900 has a fixed collectionmagnification, which simplifies and lowers the cost of the system. Whenthe illumination magnification is increased, as described above, thespot size and scan velocity are decreased. Although decreased spot sizecorresponds to increased sensitivity, decreased velocity corresponds tolower throughput. In the illustrated system 900, the obliqueillumination DOE 930 b is placed after the illumination magnifier 927 b(and there is similar placement between the magnifier 927 a and DOE 930a in the normal incident channel). This DOE 930 b placement allows themagnified spot to have a same center position, regardless of the spot'ssize change. Although the nominal centers of each scan line is the samewith different magnifications, the scan length changes. As describedfurther above, this position of the DOE 930 b ensures that the centerscan position for all the spots is unchanged with changes inillumination magnification. Spot size and velocity, however, do changewith illumination magnification changes. A large AOD can also be used toprovide scanning in only a center portion of the AOD for a large spot,while providing scanning across the entire AOD for a smallest spot asfurther described above.

FIG. 10A is a diagrammatic side view of a spot separator assembly havinga slit and prisms for receiving output beams from multiple illuminationareas in accordance with a specific implementation of the presentinvention. In this example, the spot separator assembly is a pair ofrazor blades having a first edge portion 1002 a and a second edgeportion 1002 b which together form a gap or slit 1002 c. Each outputbeam may be focused to a point 1004 a that is within this gap. A singleoutput beam 1004 with a focus point 1004 a is shown in FIG. 10A althoughthe system will typically be configured to collect and isolate thescattered, diffracted, and reflected light from multiple, simultaneousscanning spots.

FIG. 10B is a diagrammatic representation of the spot separator andprisms as viewed in direction B-B of FIG. 10A. As shown in both FIGS.10A and 10B, the output beam forms a cone that focuses to a point 1004 athat is positioned where the razor edge portions 1002 a and 1002 b havea narrowest width in gap 1002 c. The output light 1004 passes throughthe razor gap 1002 c and goes out of focus to then fill prism 1006 a. Inone embodiment, each prism has a side that is adjacent to the bottom ofthe razors 1002 a and 1002 b. Each output beam separately fills aparticular prism and then is output from the prism to an adjacent fiberoptics element (984 c of FIG. 9E). That is, individual prisms arepositioned to capture individual scan spots. In one embodiment, thereare 9 prisms for each channel positioned behind the slit 1002 c.

Additionally, the position of the prisms or apertures may be adjustableto accommodate for distortion (illumination or collection opticsinduced) of the inspection system, which causes the spacing between thespots as viewed by the sensor modules 938 a, 938 b, and 938 c to benon-uniform.

Prism and razor blade alternatives may include a DOE, such as areflective or transmissive grating, under each collected spot positionin place of each prism, to reflect or transmit the output light for eachspot to a corresponding fiber optic and/or sensor. Each grating or setof gratings would include a set of diffractive features for diffractinglight from the imaged spots towards the sensor, whereas light betweenthe spots (and between the diffractive patterns) is not diffractedtowards the sensors. In another example, a substantially opaque printedpattern may be formed on a substantially transparent substrate(s) toform a slit. For instance, the printed pattern may be formed from acoating, which is deposited on a glass substrate.

The collections channels may alternatively or additionally include aseries of mirrors for providing randomization of the collected light.For instance, each prism may be replaced by a series of mirrors. One ormore mirrors may be placed in a position that corresponds to each prismfacet (e.g., 1020 a, 1020 b, 1020 c, and 1020 d). Optical fibers or acombination of mirrors and fibers may also replace the prisms.

Other suitable spot separators may include a mask with slits for theindividual scanned spots. FIG. 10C is a diagrammatic representation of amask type spot separator 1050 in accordance with a specificimplementation of the present invention. For example, the mask can beformed from a foil material with apertures (e.g., 1052 a and 1052 b)that are each shaped to enclose a single spot scan (e.g., 1051 a and1051 b). For instance, a prism may sit beneath each aperture (e.g., 1052a, 1052 b). The mask separator may replace the razor portions andoptionally combined with prisms, fibers, and/or mirrors as describedabove. One or more masks can be positioned in each channel.

A grating may be added to the mask of FIG. 10C to diffract the lightfrom each spot into a prism or fiber or other collection assembly. FIG.10D is a diagrammatic representation of a mask type spot separator withgratings in accordance with an alternative embodiment of the presentinvention. A top view of the mask 1056 and a side view of the mask 1058are shown. Each aperture has gratings (e.g., 1054 a and 1054 b). In thisexample, the gratings are aligned vertically across each aperture (asseen from top view 1056) so as to diffract the incoming light (e.g.,1046 a) in a down direction into each prism, as opposed to the nextprism. For instance, incoming spot light 1046 a is diffracted by grating1054 a down into prims 1048 a. The grating can be reflective (diffractlight up) or transmissive (as shown).

FIG. 10E illustrates a top view of a mask type spot isolator withgratings that are oriented in different directions in accordance withanother embodiment of the present invention. For example, aperture 1060a has a vertical grating; aperture 1060 b has an angled grating; andaperture 1060 c has an inversely angled grating. These different gratingorientations diffract light down/up, down/up to the left, or down/up tothe right.

An apodization mechanism may be added to the aperture assembly (934a˜934 c) to control diffraction and, thus, spot-to-spot crosstalk. FIG.10F is a diagrammatic representation of an aperture having serratedteeth and is formed from two masks with serrated teeth in accordancewith one embodiment of the present invention. FIG. 10G shows both idealand non-ideal aperture teeth. Ideally, the aperture teeth would havesharp vertical points (1080 a) and dips (1080 b) to diffract the lightaway from the optical axis (e.g., to the left and right, rather thanforward). However, the fabrication of the aperture teeth may result inrounded portions, such as dip 1082, which would diffract some unwantedlight towards the prisms. As shown in FIG. 10F, a first mask (dottedline) having rounded dips may be overlaid with a second mask (dashedline) also having rounded dips. The supposition of the two masks is thesolid line that has sharp non-rounded features. In the longitudinalcollection channels, the teeth may be oriented perpendicular to theoptical axis and in a plane that is parallel to the wafer surfacenormal. In the normal collection channel, the teeth may be perpendicularto the optical axis and in a plane that is perpendicular to the wafersurface normal.

Any suitable type of apodization mechanisms may be utilized at eachaperture stop. Variable transmission coatings can be deposited onto atransparent substrate, such as glass, to provide apodization. Differentpatterns (e.g., dots, triangles) may be printed so as to form graduateddensities at the edge of the aperture. Different densities of theprinted patterns may transmit, reflect, or diffract light to performapodization in the aperture. All printed patterns and coatings may beformed to provide a variety of transmission profiles (linear, cosine,Gaussian, etc.) that can be utilized to control crosstalk. Severalapodization techniques and mechanisms are further described in U.S. Pat.No. 5,859,424 issued 12 Jan. 1999 by Adam E. Norton et al., which patentis incorporated herein by reference in its entirety.

A Fourier filter may also be placed at the aperture stop of eachcollection channel so as to block particular diffraction spots or noiseor to isolate certain signals. The Fourier filter can be configured toselectively block portions of the light at the aperture stop. FIG. 10Hillustrates a Fourier filter in the form of closely spaced pin-likestructures that can be dropped down across the aperture to blockparticular portions of the light in accordance with one embodiment ofthe present invention. As shown, pins 1086 are dropped into the apertureto block a corresponding portion of the collected light.

FIG. 11A is a top view representation of the segmented longitudinalimaging aspect of one embodiment of the present invention. As shown,multiple spots 1110, 1112, and 1114 are scanned across the sample 902along scan lines 1111, 1113, and 1115, respectively. A first sidechannel 1102 a receives and separates longitudinal imaged scan spots1110 a, 1112 a, and 1114 a in corresponding scan directions 1116 a, 1118a, and 1120 a, respectively. Likewise, a second side channel 1102 breceives and separates longitudinal imaged scan spots 1110 b, 1112 b,and 1114 b in corresponding scan directions 1116 b, 1118 b, and 1120 b,respectively. These imaged scan spots are sensed by correspondingsensors. Image scan spots 1110 a, 1112 a, and 1114 a are sensed by, forexample, sensor modules 1104 a, 1106 a, and 1108 a, respectively.Likewise, image scan spots 1110 b, 1112 b, and 1114 b are sensed by, forexample, sensor modules 1104 b, 1106 b, and 1108 b, respectively.

The grazing angle of each beam may produce an elliptical spot on thewafer surface, having a major axis perpendicular to the scan line. TheAOD causes each spot to scan across a short scan line equal in length tothe length of scan line to produce reflected and scattered light. FIG.11B illustrates one example of the scanning direction of the stage andAOD with respect to three spots. It is noted that the size of the spotsand scans are exaggerated with respect to the wafer 1120 to betterillustrate this embodiment. In this example, the AOD produces a singlescanning spot, and a 3×31 DOE generates 3 spots: Spot1, Spot2, andSpot3. The propagation of the chirp packet through the chirp AOD causesall 3 spots to scan simultaneously across wafer 1120 in AOD scandirection 1122. Spot1 scans from wafer position 1 to 2; Spot2 scans fromwafer position 3 to 4; and Spot3 scans from wafer position 5 to 6. Whilethe AOD scans each spot, the xy stage is moving in an XY stage scandirection 1124 so that the next sweep of the AOD begins with waferpositions 7, 9, and 11 and completes at positions 8, 10, and 12.

FIG. 11C is a diagrammatic representation of interleaved scanning withXY stage motion in accordance with one embodiment of the presentinvention. It is noted that the size of the spots and scans areexaggerated with respect to the wafer to better illustrate thisembodiment. In this example, three spots are scanned in AOD scandirection 1132 from the bottom to top of three initial ribbons 1136 a,1136 b, and 1136 c. Upon reaching the border of the first scan portions(bottom to top of each ribbon), the AOD resets and positions the spotfor the next raster scan from left to right. The continuous motion ofthe XY stage in a direction perpendicular to the scanning spot directionnow positions the beam to a new area to be inspected. For instance, thestage also moves so that the three spots are simultaneously scanned inXY stage scan direction 1134 a (left to right) along ribbons 1136 a,1136 b, and 1136 c, respectively. After the first three ribbons 1136a˜1136 c are scanned, the stage steps in direction 1140 a to move to asecond set of ribbons 1137 a, 1137 b, and 1137 c that will beinterleaved between the first set of ribbons 1136 a˜1136 c. After thissecond set of ribbons are completed (e.g., scanned in direction 1134 b),the stage can then step in direction 1140 b to scan a third set ofribbons 1138 a, 1138 b, and 1138 c with the three spots. After thisthird set of ribbons 1138 a˜1138 c are scanned, the stage can then stepin direction 1140 c to the last set of ribbons 1139 a, 1139 b, and 1139c.

FIG. 11D is a diagrammatic representation of interleaved scanning withXY stage motion in accordance with an alternative embodiment of thepresent invention. In this example, the first set of ribbons 1136 a˜1136c are still initially scanned. However, the stage steps in direction1150 a for a greater distance to scan second ribbons 1140 a, 1140 b, and1140 c. The stage then steps in direction 1150 b to scan a third set ofribbons 1142 a, 1142 b, and 1142 c. Finally, the stage steps indirection 1150 c to scan a fourth set of ribbons 1144 a, 1144 b, and1144 c. The topmost ribbon (1144 c) and bottommost ribbon (1136 a) maybe thrown out of the analysis.

Longitudinal imaging of multiple scanned spots along the optical axiscan result in minimization of certain optical aberrations. For instance,optical aberrations that require a lateral field (e.g., coma) will beminimized when imaging along the optical axis. The longitudinal imaging(with no lateral field) combined with interleaved scanning (with reducedimaging requirements) provides a simple and inexpensive way to supportboth a large collection angle (NA>0.9) and large FOV (FOV>1 mm). Morespecifically, certain system embodiments provide multiple collectionchannels (two longitudinal and normal) that simultaneously collect frommultiple segmented scan lines.

A large FOV combined with a large collection NA may result in a highthroughput, high performance system. A large FOV allows the ability toreach high throughputs. Precision XY stages are limited in terms of howfast they can move the sample under inspection. A large FOV systeminspects large portions of the wafer with a minimum of XY stageoperation. In addition, a large FOV and, hence, high throughput systemenables the user to run higher resolution modes at the same speed.

Certain collection channel embodiments collect over a large solid angleand may be associated with increased sensitivity. A large collectionsolid angle ensures that the system collects the signal of interest. Alarge solid angle of collection also enables the suppression of noise(roughness and other sources). Additionally, the large solid angleenables the system to utilize features such as flexible apertureconfigurations to select regions with high signal and low noise.

Large solid angle of collection and large FOV are a result of thesegmentation of the system into multiple optical channels, includinglongitudinal channels. Additionally, the spot isolation mechanismsdescribed herein reduce cross-talk between spots, which would otherwisecontribute to noise and false defect detection.

FIG. 12 is a flow chart illustrating a general procedure 1200 forinspecting a sample using a longitudinal system in accordance with oneembodiment of the present invention. Initially, multiple incident beamsmay be scanned over separated scan line portions in operation 1202. Forinstance, the normal and oblique incident channels of FIGS. 9A˜9E may beused to generate normal and oblique incident beams in each illuminationchannel.

Output light scattered from the separated scan lines on the samplesurface in response to the incident beams may then be received andseparated in operation 1204. For example, the side and normal collectionchannels may be used to receive and separate light that is reflected,diffracted, and scattered from the sample scan line portions. Eachseparated output light beam may then be directed to a separate sensor inoperation 1206. In the embodiment of FIG. 9A˜9E, each separated beamfrom each channel 931 a˜931 c may be collected by such channels sensors.

Defects may then be detected or sample characteristics measured based onthe sensed light in operation 1208. The detected images (or signals) maygenerally be analyzed to determine whether defects are present on thesample. For example, the intensity values from a target die are comparedto the intensity values from a corresponding portion of a reference die(or generated from a design database), where a significant intensitydifference may be defined as a defect. These inspection systems mayimplement any suitable inspection technology, along with thelongitudinal imaging mechanisms described herein. By way of examples,brightfield and/or darkfield optical inspection mechanisms may beutilized. The mechanisms of the present invention may also beimplemented within a scanning electron microscopy system.

Each detected image may also be input to a defect (e.g., image)processor (e.g., 950). Defect processor may include mechanisms forprocessing the received data, such as buffering, compressing, packing,filtering noise, generating images based on the input signal, analyzingimages to detect defects on the sample, etc. The majority of defects maybe found by detecting contrast, defined as the ratio of the intensitiesin the target and reference dies, rather than by threshold, which isdefined as the difference between the intensities.

The longitudinal collection systems described herein may be implementedon various specially configured inspection or metrology systems, such asthe one schematically illustrated in FIGS. 9A˜9E. In certainembodiments, a system for inspecting or measuring a specimen includesvarious controller components for implementing the techniques describedherein. The controller may be implemented by any suitable combination ofhardware and/or software, such as a processor, memory, programmabledevice or field programmable gate array (FPGA), etc.

The inspection system may be associated with a computer system that isconfigured (e.g., with programming instructions) to provide a userinterface (e.g., on a computer screen) for displaying resultantinspection characteristics. The computer system may also include one ormore input devices (e.g., a keyboard, mouse, joystick) for providinguser input, such as changing detection parameters. In certainembodiments, the computer system is configured to carry out inspectiontechniques in conjunction with other inspection components, such ascontroller 950, detailed herein. The computer system typically has oneor more processors coupled to input/output ports, and one or morememories via appropriate buses or other communication mechanisms.

Because such information and program instructions may be implemented ona specially configured computer system, such a system includes programinstructions/computer code for performing various operations describedherein that can be stored on a computer readable media. Examples ofmachine-readable media include, but are not limited to, magnetic mediasuch as hard disks, floppy disks, and magnetic tape; optical media suchas CD-ROM disks; magneto-optical media such as optical disks; andhardware devices that are specially configured to store and performprogram instructions, such as read-only memory devices (ROM) and randomaccess memory (RAM). Examples of program instructions include bothmachine code, such as produced by a compiler, and files containinghigher level code that may be executed by the computer using aninterpreter.

Other system embodiments may include additional oblique side channels todetect light scattered from the sample surface by a plurality ofdetectors, in addition to longitudinal side channels. These additionalcollector channels can be arranged to collect light over a fixed solidangle, dependent upon, inter alia, the elevational and azimuthal angleof the channel. Those of ordinary skill in the art will readilyrecognize that the number and location of the collector channels and/ortheir collection solid angle may be changed in various alternativeembodiments without departing from the scope of the invention. Severalsystem embodiments that include additional collection channels arefurther described in U.S. patent application Ser. No. 13/898,736,entitled “Image Synchronization of Scanning Wafer Inspection System”,filed 21 May 2013 by Kai Cao et al., which application is incorporatedherein by reference in its entirety.

A bright field reflectivity/autoposition channel can also be positionedin front of the oblique incident beam to collect specularly reflectedlight. The bright field signal derived from this channel carriesinformation concerning the pattern, local variations in reflectivity andheight. This channel is sensitive to detecting various defects on asurface. For example, the bright field signal is sensitive torepresenting film thickness variations, discoloration, stains and localchanges in dielectric constant. The bright field signal could be used toproduce an error height signal, corresponding to a variation in waferheight, which is fed to a z-stage to adjust the height accordingly. Aseparate autofocus can also be inserted into the system to image throughthe normal collection channel. Finally, the bright field signal can beused to construct a reflectivity map of the surface. In one embodiment,this channel is basically an unfolded Type I confocal microscopeoperating in reflection mode. It is considered unfolded because theilluminating beam and reflected beams, here, are not collinear, ascompared with a typical reflection confocal microscope in which theilluminating and reflected beams are collinear.

The brightness of a scan line produced by a system as described abovemay be calibrated by scanning a specimen of uniform reflectivity. Lightscattered from different positions along the final scan line may becollected and measured. The amplitude of the drive signal applied to theprescan AOD may then be modulated as needed to produce a scan line ofmeasured uniform brightness at the specimen. This calibration maycompensate not only for attenuation in the chirp AOD, but for any othernon-uniformities in the scanning system.

The illumination system may also include additional optical components(not shown). For example, additional optical components may include, butmay not be limited to, beam splitters, quarter wave plates, polarizerssuch as linear and circular polarizers, rotating polarizers, rotatinganalyzers, collimators, focusing lenses, mirrors, dichroic mirrors,partially transmissive mirrors, filters such as spectral or polarizingfilters, spatial filters, reflectors, and modulators. Each of theseadditional optical components may be disposed within the system or maybe coupled to any of the components of the system as described herein.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatus of the presentinvention. For example, the collection channels can be configured tosimultaneously collect from the normal incidence and oblique incidenceillumination channels. Additionally, the system may exclude a magnifierchanger. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. A system for inspecting or measuring a specimen,comprising: an illumination channel for generating and scanning aplurality of incident beams at an oblique angle to form a plurality ofspots that scan across a segmented line comprised of a plurality of scanportions of the specimen; and one or more detection channels for sensinglight emanating from a specimen in response to the incident beamsdirected towards such specimen and collecting a detected image for eachscan portion as each incident beam's spot is scanned over its scanportion, wherein the one or more detection channels include at least onelongitudinal side channel for longitudinally collecting a detected imagefor each scan portion as each incident beam's spot is scanned over itsscan portion, wherein the at least one longitudinal side channel isarranged to have an optical axis along which the detected image iscollected and which is also parallel and coincident with a plane of thespecimen or an image plane of the specimen.
 2. The system of claim 1,wherein the one or more detection channels include a first longitudinalside channel for longitudinally collecting a first plurality of detectedimages for the scan portions, a second longitudinal side channel forlongitudinally collecting a second plurality of detected images for thescan portions, wherein the first longitudinal side channel is positionedopposite of the second longitudinal side channel.
 3. The system of claim1, wherein each of the at least one detection channels are configured tocollect its detected images in parallel for the plurality of scanportions.
 4. The system of claim 3, wherein the illumination channelincludes a normal illumination sub-channel for generating and scanning afirst set of the plurality of incident beams to contribute to theplurality of spots that scan across the plurality of scan portions ofthe specimen and an oblique illumination sub-channel for generating andscanning a second set of the plurality of incident beams to contributeto the plurality of spots that scan across the plurality of scanportions of the specimen, and wherein the one or more detection channelsinclude a first longitudinal side channel for longitudinally collectinga first plurality of detected images for the scan portions, a secondlongitudinal side channel for longitudinally collecting a secondplurality of detected images for the scan portions, and a normalcollection channel for collecting a third plurality of detected imagesfor the scan portions, wherein the first longitudinal side channel ispositioned opposite of the second longitudinal side channel.
 5. Thesystem of claim 4, wherein the normal illumination sub-channel and theoblique illumination sub-channel each include a waveplate for providinga plurality of polarizations.
 6. The system of claim 5, wherein thefirst and second longitudinal side channels and the normal collectionchannel each include an analyzer for capturing selected polarizations.7. The system of claim 4, wherein the normal illumination sub-channeland the oblique illumination sub-channel each include an acousto-opticaldevice (AOD) for scanning the incident beams, the system furthercomprising a controller for modulating an amplitude of a drive signal toeach AOD for controlling its corresponding incident beams' power.
 8. Thesystem of claim 4, wherein the normal illumination sub-channel and theoblique illumination sub-channel each include an apodization element toprovide a variety of transmission profiles.
 9. A system for inspectingor measuring a specimen, comprising: an illumination channel forgenerating and scanning a plurality of incident beams obliquely to forma plurality of spots that scan across a segmented line comprised of aplurality of scan portions of the specimen; and one or more detectionchannels for sensing light emanating from a specimen in response to theincident beams directed towards such specimen and collecting a detectedimage for each scan portion as each incident beam's spot is scanned overits scan portion, wherein the one or more detection channels comprises alongitudinal side channel for longitudinally collecting a plurality ofdetected images for the scan portions, wherein the longitudinal sidechannel comprises: a front lens arranged for receiving output beams thatare scattered from the scan portions and directing such output beamsthrough a Fourier plane towards a back lens, the back lens arranged forreceiving and directing the output beams towards a sensor module, thesensor module arranged for separately sensing the output beams from theback lens, and an optics element arranged for receiving the output beamsfrom the front lens, spatially filtering portions of the output beams atthe Fourier plane, and directing the output beams to the back lens,wherein the optics element includes an aperture having serrated teethpointed perpendicular to an optical axis for controlling diffraction.10. The system of claim 9, wherein the serrated teeth are formed fromtwo overlaid masks with serrated teeth to cover rounded portions of theteeth in each mask and to form non-rounded serrated teeth.
 11. Thesystem of claim 9, wherein the optics element includes a plurality ofpins that are independently movable to drop down into each aperture andselectively block noise, isolate signals, or block one or morediffraction spots.
 12. The system of claim 9, wherein the illuminationchannel comprises a magnifier changer.
 13. The system of claim 12,wherein the illumination channel includes a scan mechanism that isconfigured to sweep the incident beams across equally sized scanportions on the sample.
 14. The system of claim 12, wherein thelongitudinal side channel includes a magnifier changer to match amagnification of the magnifier changer of the illumination channel. 15.The system of claim 9, wherein the sensor module comprises: a first andsecond razor portion forming a gap there between arranged to receive afocus point for each of the output beams, a plurality of prisms that areeach positioned at each of the output beams' focus point to separatelyreceive and direct the output beams to a plurality of fiber elements,the fiber elements arranged to separately receive and direct the outputbeams from the plurality of prisms to a plurality of focusing elements,the plurality of focusing elements being arranged for individuallyfocusing the output beams onto a plurality of sensor elements, and theplurality of sensor elements being positioned for individually sensingthe output beams.
 16. The system of claim 15, wherein the prisms aremovable to compensate for distortion.
 17. The system of claim 9, whereinthe sensor module comprises: a first and second razor portion forming agap there between arranged to receive a focus point for each of theoutput beams, a plurality of mirror and/or fiber elements sets that areeach positioned at each of the output beams' focus point to separatelyreceive and direct the output beams to a plurality of focusing elements,the plurality of focusing elements being arranged for individuallyfocusing the output beams onto a plurality of sensor elements, and theplurality of sensor elements being positioned for individually sensingthe output beams.
 18. The system of claim 9, wherein the sensor modulecomprises: a mask having a plurality of apertures that each receive afocus point for each of the output beams, a plurality of prisms or setsof mirrors that are each positioned at each of the output beams' focuspoint to separately receive and direct the output beams to a pluralityof fiber elements, the fiber elements arranged to separately receive anddirect the output beams from the plurality of prisms to a plurality offocusing elements, the plurality of focusing elements being arranged forindividually focusing the output beams onto a plurality of sensorelements, and the plurality of sensor elements being positioned forindividually sensing the output beams.
 19. The system of claim 18,wherein each of the masks includes a grating in each aperture fordirecting the output beams, respectively towards the sensor elements,respectively.
 20. The system of claim 19, wherein at least some of thegratings of the masks have orientations in different directions.
 21. Thesystem of claim 19, wherein at least some of the gratings of the maskshave orientations in a same direction.
 22. A method of inspecting aspecimen, comprising: scanning multiple incident beams obliquely overseparated scan lines of the specimen; receiving and separating outputbeams scattered from the separated scan lines of the specimen inresponse to the incident beams; longitudinally directing each separatedoutput beam towards a sensor to longitudinally generate an image orsignal, wherein each separated output beam is directed along an opticalaxis along which each scan line is imaged and which is also parallel andcoincident with a plane of the specimen or an image plane of thespecimen; and detecting defects or measuring a characteristic of thespecimen based on the image or signal from each sensor.